1. Field of the Invention
The present invention relates to a process for producing sol-gel condensates by employing aqueous silica sols, and also to their use for coating of inorganic or organic substrates.
2. Description of the Prior Art
By co-condensation of SiO2 nanoparticles with alkyltrialkoxysilanes, such as methyltrimethoxysilane for example, sol-gel condensates can be obtained which, for example, can be cured on plastic surfaces to form inorganic coatings having high scratch resistance. Production of these sol-gel condensates is effected by conversion of methyltrimethoxysilanes with an aqueous dispersion of SiO2 nanoparticles (aqueous silica sol) in the presence of organic solvents, as described in U.S. Pat. No. 4,476,281 for example. The addition of organic solvents is necessary, since due to the presence of the hydrophobic alkyl residue neither the alkyltrialkoxysilanes that are employed nor the hydrolysis products and condensation products thereof are completely miscible with water.
In contrast with the sol-gel condensates that have been described based on alkyltrialkoxysilanes in which the organic residues are exclusively terminally bonded, through the use of polyfunctional organosilanes a chemical linkage of the organic residues with the inorganic network can be obtained. Polyfunctional organosilanes, as described in EP-A 0 947 520 for example, are linear, branched or cyclic monomeric oligosilanes that possess at least two silicon atoms with hydrolyzable and/or condensation-crosslinking groups, the silicon atoms each being bonded via at least one carbon atom to a linking structural unit.
Polyfunctional organosilanes in which the linking structural unit is a cyclic siloxane, e.g. cyclo-{OSi[(CH2)2Si(OH)(CH3)2]}4, are of particular interest as starting materials for the production of sol-gel condensates, since, for example, sol-gel coatings with a scratch resistance similar to that of glass can be produced therefrom (see EP-A 0 947 520). These scratch-resistant coatings are of interest, in particular, in the production of automobile coatings. Furthermore, such sol-gel coatings are distinguished by pronounced hydrophobicity, enabling their use for the production of, for example, anti-graffiti coatings or fouling-release coatings (e.g. EP-A 967 253).
With a view to producing sol-gel condensates that are suitable for the coating of surfaces, the polyfunctional organosilane is conventionally converted not on its own but in combination with metal alkoxides and/or nanoparticles. By suitable choice of the quantitative ratios, the hydrophobicity and the hardness of the resulting coating can be adjusted, depending on the requirements. In this connection it has previously been a great disadvantage that the conversion of the polyfunctional organosilanes with SiO2 nanoparticles in the presence of metal alkoxides was only possible by using solvent-containing SiO2 nanoparticle dispersions, so-called organosols (e.g. EP-A 0 947 520). Most polyfunctional organosilanes are totally immiscible with water and are consequently also incompatible with aqueous dispersions of SiO2 nanoparticles (aqueous silica sols). The production of organosols, however, is substantially more elaborate than that of corresponding aqueous silica sols that are available on the market as standard commercial products (e.g. Levasil, Bayer AG, Leverkusen).
An object of the present invention is to provide a process that enables the incorporation of aqueous silica sols into sol-gel condensates based on polyfunctional organosilanes.
It has now been found that condensates that are miscible with polyfunctional organosilanes can be obtained by reaction of an aqueous silica sol with a silicon alkoxide.
The present invention relates to a process for producing sol-gel condensates by
A1) reacting an aqueous silica sol with a silicon alkoxide and subsequently
A2) converting the condensate obtained from A1) with a polyfunctional organosilane.